1. Field of the Invention
The invention relates to a method and apparatus for detecting surface plasma waves, more particularly to an optical heterodyne surface plasma wave detecting method and apparatus.
2. Description of the Related Art
Conventional immunoassays, such as ELISA or RIA, for detecting pathogenic bacteria or virus require at least three days and involve repeated rinsing-reacting-rinsing operations. When optic fiber biosensors are in use, although the detection time can be shortened, they additionally require the aid of fluorescent markers.
A detecting method based on surface plasma resonance has been proposed heretofore to achieve fast detection without the need for fluorescent markers. As defined herein, surface plasma wave is an electromagnetic wave that oscillates at a metal surface. FIG. 1 illustrates how the phenomenon of surface plasma resonance can be realized. As shown, incident P-polarization light (TM wave) is directed by a total reflective component, such as a prism 60, to a metal film 61, thereby generating a surface plasma wave at an interface 610 between the metal film 61 and a test object on the interface 610 in a direction parallel to the interface 610. At the same time, energy of the incident P-polarization light is partly removed, thereby reducing the intensity of reflected light from the total reflective component.
By changing the incident angle (θ), the wave vector (kx) parallel to the interface 610 of the metal film 61 can approximate the wave vector (ksp) of the surface plasma wave to satisfy the following resonance condition: kx=kg sin θ=ksp, wherein kg=[ω/c](∈0)1/2, ksp=[ω/c](∈1∈2/∈1+∈2)1/2, ω is the frequency of the incident light, and ∈0, ∈1, and ∈2 are the dielectric coefficients of the prism 60, the metal film 61 and the test object (not shown), As best shown in FIG. 2, a reduction in reflectivity becomes more and more evident when the resonance condition is satisfied.
With further reference to FIG. 3, a change in the dielectric coefficient (∈z) or refractive index will lead to a shift in the resonance angle. By measuring this shift, a change in physical property, such as the refractive index, concentration, etc., can be observed. The change in physical property can be similarly observed by measuring a change in the intensity of the reflected light under fixed incident angle conditions. Accordingly, the time-varying change in physical property can be also monitored to result in the curve of FIG. 4.
The above detection methods are widely used in the fields of biomedicine and material chemistry. However, regardless of whether the change in the intensity of the reflected light or the change in the resonance angle is relied upon in the detection of physical properties, the aforesaid conventional detection methods are still unsatisfactory in view of their relatively low sensitivity.